The present invention is directed to a method for producing superconducting oxide compounds which are formable into fibers and other desired shapes. The present invention is also directed to an improved method for making superconducting oxide compounds with advantageous homogeneity and current-carrying capacity.
Until recently, known superconducting materials have been metallic materials which exhibit superconducting properties only at temperatures close to absolute zero. However, recently a novel class of superconducting materials which take the form of ceramics, or mixed metal oxides, have been discovered, some of which exhibit superconductivity above the temperature of liquid nitrogen, 77xc2x0 K. (xe2x88x92321xc2x0 F.), at ambient pressure, which signifies the ability to prepare and maintain superconductive materials now in virtually any laboratory. These mixed metal oxides are made typically by sputtering the appropriate metals and metal oxides onto a substrate and sintering to form the requisite ceramic structure. Another method involves coprecipitating the appropriate metals from aqueous solutions of their nitrate salts, then heating the precipitate at 900xc2x0 C. to 950xc2x0 C. to form the appropriate ceramic structure.
One of the disadvantages of the sintering method is that it produces islands of the correct compound composition for superconductivity, interconnected by areas where the stoichiometry is not optimized for superconductivity. This problem of nonhomogeneity may partially account for the low current-carrying characteristics of the mixed metal oxide superconductors. If these superconducting materials are to have a broad base industrial application, methods need to be devised to increase their current carrying capacity.
Another problem is that the ceramic material is brittle, hard and difficult to handle without damaging the ceramic, and is particularly difficult to form into a wire or fiber, which is useful for high-current applications.
For small-scale applications, such as for use as components in electronic devices, the low current-carrying capacity problem may be solved by fabricating the ceramic materials in the form of single crystals. However, the technology for making large single crystals suitable for high-current industrial uses is not practically available.
It is therefore an object of the present invention to provide a method for producing mixed metal oxide superconducting materials which can be made into virtually any desired shape or form and which, in particular, may be cast or extruded.
It is another object of the present invention to provide a method for making mixed metal oxide superconductors characterized by advantageous homogeneity.
It is a further object of the present invention to provide a method for making mixed metal oxide superconductors whereby precise dopant concentration can be tailored to create optimum oxide charge states in the superconductor.
It is yet another object of the present invention to provide novel mixed metal oxide superconductors according to the processes disclosed hereinafter.
These and other objects of the present invention will be apparent from the following description of the preferred embodiments of the invention, the appended claims, and may from the practice of the invention.
The present invention provides a method for preparing solid superconducting mixed-metal oxides in a predetermined shape and form, comprising the steps of providing a solution of salts of the metals contained in the desired superconducting mixed-metal oxide of predetermined composition, wherein each of the salts is present in an amount necessary to provide the predetermined stoichiometric amount of each respective metal required in the desired superconductive mixed-metal oxide; and wherein the counterions, or hydrolysis products thereof, of the metal ions for each of the salts in the solution are removable from the solution by evaporative methods; subjecting the solution to hydrolyzing conditions and removing the counterions and/or hydrolysis products thereof, and a substantial portion of the solvent, from the solution by evaporative methods; converting the metal ions to a mixed metal oxide precursor of the superconducting mixed metal oxide; peptizing the mixed metal oxide precursor to form a viscous polymeric sol; forming the viscous polymeric sol into a predetermined shape or form and heat-setting the sol to a flexible, ductile gel; firing the heat-set gel in the presence of oxygen at a temperature and for a period of time sufficient to oxidize and volatilize any remaining vapors and organic materials from the gel and to form the solid superconducting mixed metal oxide. Novel superconducting mixed metal oxides according to the present invention are also provided, as well as novel viscous, castable, extrudable mixed metal oxide precursors.
The present invention provides a method for producing mixed metal oxide superconducting materials of a predetermined shape, such as tape, fibers, and coatings. In the context of the description of the invention the term sol will have its accepted technical meaning: a colloidal solution. The term gel will have its accepted technical meaning: a colloidal solution of a liquid in a solid. The starting materials for the method are soluble salts, in particular the soluble organic salts, of the metals which comprise the final mixed metal oxide superconductor. The metal salts may be soluble in water, or water miscible alcohol, mixtures thereof, or any other water-miscible solvent which can be removed by evaporation without a reaction which is deleterious to the formation of the desired mixed metal oxide structure. If an appropriate soluble salt of a desired metal is not readily available, but is available as an insoluble metal halide, the metal may be incorporated, alternatively, as a colloidal gel by reacting the metal halide (such as a metal chloride) with water to make a colloidal metal hydroxide. Such a colloidal metal hydroxide may be separated from an ammonium chloride solution, then reacted with the sol containing hydroxides and/or oxides of the other metals to be incorporated into the mixed oxide superconductor.
While not intending to be limited by any particular theory, it is believed that the method of preparation of the mixed metal oxides of the present invention permit the formation of the appropriate metal oxide structure to occur on a colloidal level among particles the size of about 1-10 nm in diameter in the proper stoichiometry and lattice conformation, valence ratio and phase relationships, thereby producing compounds which are believed to be more homogeneous than a mixed metal oxide superconductor of similar composition made by sputtering and sintering of mixed-metal oxide or carbonate powders. Sputtering of metal oxide or carbonate powders allows mixing of particles on the order of 1-10 xcexcm in size, which are sintered more slowly than the colloidal particles according to the method of the present invention.
A solution is first prepared containing soluble salts of the metals ultimately required in the mixed metal oxide superconductor. These salts are preferably soluble in water or in a water-miscible alcohol, such as methanol, ethanol, isopropanol, ethylene glycol and the like. The appropriate salts include those which provide, as a counterion to the metal ion, an ion which is removable by evaporative methods, or at least the hydrolysis product of which is removable by evaporative methods. This thus includes the organic counterions such as the acetates and formates, as well as counterions which evolve as gases at an appropriate pH, such as the carbonates. To assist in solubilizing the metal salts, polyhydroxy compounds, such as, ethylene glycol, and organic acids, such as citric acid, malonic acid, acetic acid, and the like, may be added to form the metal salt solution. These polyhydroxy compounds and organic acids retain metal salts in solution, since some salts would precipitate under subsequent distillation conditions. Exemplary salts of those metals which comprise mixed metal oxide superconductors include, but are not limited to:
It is contemplated that in some instances an appropriate soluble salt of a desired metal may not be readily available. In such cases an available insoluble metal halide, such as the metal chloride, may be used to prepare a colloidal metal hydroxide which, in turn, may be later added to the peptized hydroxide sol containing the other metals required for the making of the mixed oxide superconductor. For example, a metal chloride may be reacted with water to form a colloidal metal hydroxide. The colloidal metal hydroxide may be separated from an ammonium chloride solution and then added to the sol containing the other hydroxides or oxides of the other metals. Exemplary halide salts which may be utilized in this manner include, but are not limited to:
barium chloride
barium fluoride
strontium fluoride
strontium iodide
strontium bromide
lanthanum chloride
gadolinium fluoride
erbium iodide
scandium iodide
dysprosium iodide
proseodymium chloride
After preparation of the solution of soluble metal salts, if water is not already present in the solution, water is then added and the solution is subjected to hydrolyzing conditions whereby the counterions of the metal ions, or their hydrolysis products, are converted to moieties which are removable, by evaporative methods, such as by evolution of gas, or by evaporation of alcohols or organic acid. This may normally be done by distillation whereby the organic products are removed from the metals along with a substantial portion of the organic solvent and water. Subsequent to or simultaneous with distillation, the metals are converted by heating to oxides to form a mixed metal oxide precursor for the superconductor material.
The mixed metal oxide precursor, which is then typically a homogeneous semi-solid, is peptized to a sol, or fluid colloidal system, usually by addition of a strong mineral or organic acid, such as concentrated nitric acid, hydrochloric acid, lactic acid, and the like. This peptization step is usually conducted by heating at a temperature of less than about 100xc2x0 C. At this time, metal colloidal gel, prepared by reacting metal halide and water, may be added to provide the metal or metals for which there were no available soluble salts. During this peptization process, the polymeric chains of the inorganic oxides are then formed.
Heating this sol produces a thick, viscous gel which can then be cast into thin strips, extruded, or drawn, as continuous or discontinuous fibers, into thin monofilamentary fibers or multifilamentary tows. The gel can also be diluted and sprayed as a chemically homogeneous coating, for example, on a resonance cavity of a particle accelerator. Upon forming the gel into its desired form either as continuous fibers, discontinuous fibers, tape, coating, or otherwise, the gel is heat-set, usually by contact with a hot flowing air environment, typically at about 80-120xc2x0 C. The resultant hard-gelled mixed-metal oxide is ductile, flexible and handleable, and thus is an improvement over products made by the sintered powder method.
As a final step, the mixed metal oxide in its desired hard-gelled shape, is fired at a temperature and for a period of time sufficient to oxidize and volatilize any remaining vapors and organic materials, thereby leaving an intact, dense, mixed metal oxide ceramic superconductor in its desired superconducting form. While this period of time will vary, usually one to six hours will suffice. Usually, the firing temperature will be in the range of about 900-1100xc2x0 C. The preferred firing temperatures are in the range of 900-1000xc2x0 C., most preferably at about 950xc2x0 C.
Although not required, in some instances it is desirable to provide a final high-pressure (1 atm. or greater) oxygen heat treatment at a temperature of about 1000-1100xc2x0 C. to partially compensate the p-type semiconductor mixed metal oxide through a slight oxygen deficiency on the oxygen sublattice. Such a high-pressure oxygen firing would be desirable, for instance, to make sensitive components for solid state electronic devices.
The final mixed metal oxide superconductors produced according to the present invention include, but are not limited to, those having the formula A2GCu3O7xe2x88x92x, wherein A is any Group II metal, with barium and strontium being preferred; and G is any Group III metal or lanthanide, with lanthanum and yttrium being the preferred Group III metals and gadolinium and erbium being the preferred lanthanides. In the aforementioned formula, x may vary from about 2 to 3. Another metal oxide superconductor, when yttrium is present, is represented by the formula of [Y1xe2x88x92xBax]2CuO4xe2x88x92d, wherein d is zero or any positive number less than 4 and x is zero or any positive number less than 1. Particularly preferred mixed metal superconductors are those which are the mixed metal oxides of yttrium-barium-copper and gadolinium-barium-copper since these have been reported to exhibit superconducting properties above the temperature of liquid nitrogen at atmospheric pressure. It will, of course, be appreciated that the superconducting properties of mixed metal oxides depend at least in part on various lattice parameters (packing, atomic sizes, etc.), the valence ratios among the atomic metal components and the phase interrelationships present in the structure. To the extent that these can be controlled by using precise amounts of the various metals forming the oxide, the present invention is an improvement over the sputtering method. Moreover, the precise dopant level concentrations of any particular metal can be exactly tailored so that the precise oxide charge state may be produced to optimize the superconducting properties. Such precision and control in formulating a superconductor is not available on a predictable basis by the sputtering process.
By virtue of their improved homogeneity, it is also believed that the superconductor products made according to the present invention are novel and advantageous as compared to the prior art high-temperature mixed metal oxide superconductors which are made by sputtering and sintered powder methods.